Sight or gauge glass level indicators are widely used to provide a visual indication of water level in for instance, steam boilers. For safety reasons it is compulsory, in most countries, for steam boiler installations to be provided with sight glasses of the aforementioned type, irrespective of whether such boiler installations comprise level sensing and/or controlling apparatus. This is considered essential because automatic sensing devices are subject to mechanical and/or electrical failure and in the event of such failure, it is argued, it should still be possible to visually verify the level indicated by a sensing device.
Various types of level sensing devices are known to the applicant. Thus for instance, float operated, capacitative, reflector type, conductivity probe type and other types of level sensing devices are available on the market. However, many of these devices are incapable of being used in conjunction with a visual sight glass.